Image display apparatuses using light emitting diodes (LED) as light emitting elements have been proposed. An image display apparatus of this kind is constructed by including a plurality of LEDs that emit red (R) light, green (G) light and blue (B) light, illumination optical systems that receive lights from the plurality of LEDs, light valves having liquid crystal display panels that receive lights from the illumination optical system, a color combining prism that combines lights of the different colors from the light valves, and a projection optical system for projecting light from the color combining prism onto a projection plane.
With respect to the image display apparatus constructed as described above, there is a demand for preventing the occurrence of optical loss in the optical paths from the light emitting elements to the projection optical system as much as possible in order to increase the luminance of projected images.
The liquid crystal display panels and the color combining prism in the above-described components have polarization dependences. From the viewpoint of improving the efficiency of the optical system, therefore, it is desirable that each light emitting element emit linearly polarized light.
Also, as described in non-patent literature 1, there is a constraint to the efficiency of the optical system due to the etendue determined by the product of the area of the light emitting element and the radiation angle. Specifically, light from the light emitting element is not utilized as projection light, unless the value of the product of the area of the light emitting element and the radiation angle is no more than the product of the area of the entrance surface of the light valve and the acceptance angle determined by the F number of the illumination optical system.
Therefore, the pending problem with image display apparatuses using LEDs is to increase the luminance of each light emitting element is to be increased without increasing the area of the light emitting element in order to reduce the etendue of light emitted from the light emitting element.
With respect to the image display apparatus constructed as described above, there is also a demand for making the uniformity of the light intensity distribution on a screen as high as possible in order to limit luminance unevenness in projected images.
Patent literature 1 (JP2009-111012A) discloses a semiconductor light emitting element designed to emit light having a large polarization ratio and having a nonpolar plane as a principal plane of crystal growth.
Patent literature 2 (JP2007-109689A) discloses a light emitting element designed to reduce the etendue and emit linearly polarized light with high polarization conversion efficiency and having a light emitting portion that is provided on a reference plane and that emits light, a λ/4 phase plate provided on the exit surface side of the light emitting portion, a reflection-type polarizing plate that is provided on the exit surface side of the λ/4 phase plate and that allows polarized light in a first oscillation direction to pass therethrough and that reflects polarized light in a second oscillation direction perpendicular to the first oscillation direction, and an optical portion that is provided on the exit surface side of the reflection-type polarizing plate and in which the refractive index is periodically changed in two directions in a plane parallel to the reference plane.
In this light emitting element, light reflected by the reflection-type polarizing plate is reflected by a reflective electrode provided on the side of the light emitting portion opposite from the exit surface of the light emitting portion, and exits and returns through the λ/4 phase plate so that the oscillation direction is changed through 90 degrees, again enters the reflection-type polarizing plate and passes through the reflection-type polarizing plate.
The applicant of the present application proposed, in Japanese Patent Application No. 2009-243367, an invention devised to realize a light emitting element that has high efficiency and that emits linearly polarized light enabling increasing the luminance.
The light emitting element proposed in Japanese Patent Application No. 2009-243367 has an active layer in which light is produced, a polarizer layer having a first region that allows a polarization component in a first direction in the light produced in the active layer to pass therethrough and reflects other polarization components, and a second region that allows a polarization component in a second direction perpendicular to the first direction to pass therethrough and reflects other polarization components, a wave plate layer having third and fourth regions that receive lights from the first and second regions and cause the received lights to exit as lights in the same polarized state, and a reflective layer that reflects the lights reflected by the first and second regions.
In the above-described light emitting element, the polarization component in the first direction that has entered the first region of the polarizer layer and the polarization component in the second direction that has entered the second region in the light produced in the active layer pass through the polarizer layer, while the polarization component in the second direction that has entered the first region of the polarizer layer and the polarization component in the first direction that has entered the second region are reflected by the polarizer layer. The light reflected by the polarizer layer is reflected by the reflective layer and again enters the polarizer layer. The incident position at this time is different from that at the preceding time. The light that has entered the region different from that the light that entered at the preceding time passes through the polarizer layer. The light that has entered the same region as the light that entered at the preceding time is again reflected by the polarizer layer. By repeating this operation, the whole of the light produced in active layer passes through the polarizer layer in the end. The lights that have passed through the first and second regions of the polarizer layer respectively pass through the third and fourth regions of the wave plate layer to have the same polarization direction.